Polyamide is being widely used in such purposes as a hollow molded container, a film, a sheet wrapping material, engineering plastics and fibers owing to the excellent physical and mechanical characteristics thereof. Representative examples thereof include an aliphatic polyamide, such as nylon 66, and in addition, a number of kinds of polyamide have been known that use an aromatic diamine, such as p-xylylenediamine (PXDA) and m-xylylenediamine (MXDA), as a starting material to realize reduction in water absorptivity and improvement in elastic modulus.
Polyamide is relatively unstable to heat as compared to polyester and the like, and sometimes suffers gelation, yellowing and the like due to heat deterioration and thermal oxidation deterioration.
As a measure for suppressing polyamide from suffering heat deterioration, such a method is proposed that a phosphonic acid compound or a phosphorous acid compound, and an alkali metal are added to polyamide (see, for example, Patent Document 1).
As a measure for suppressing polyamide from suffering heat deterioration, such a method is proposed that a phosphinic acid compound, a phosphorous acid compound or a phosphonic acid compound, and an alkali metal are added to polyamide, and phenylenediamine and/or a derivative thereof is added to polyamide in an amount of from 0.01 to 2% by weight (see, for example, Patent Document 2).
As a measure for preventing polyamide from suffering heat deterioration at a melting point thereof or lower in a system having no oxygen present therein, such a method is proposed that a pyrophosphite salt, an amide compound of an organic phosphinic acid, a barium salt of a monoester or diester of phosphorous acid, a copper salt of a monoester or diester of orthophosphoric acid, or the like is added (see, for example, Patent Documents 3, 4, 5 and 6).
As a measure for preventing a gelled product of polyamide formed from m xylylenediamine and adipic acid from occurring, it is considered that at least one kind selected from a lubricant, an organic phosphorus stabilizer, a hindered phenol compound and a hindered amine compound is added in an amount of from 0.0005 to 0.5 part by weight (see, for example, Patent Document 7).
These measures exert effect for preventing gelation due to a network structure in polyamide, but a finer filter is being used on molding a resin for purposes requiring increased precision in recent years, which brings about such a problem that a filter is often clogged, and thus is necessarily exchanged frequently.
Patent Document 1: JP-A-49-45 960
Patent Document 2: JP-A-49-53945
Patent Document 3: JP-B-45-11836
Patent Document 4: JP-B-45-35667
Patent Document 5: JP-B-45-12986
Patent Document 6: JP-B-46-38351
Patent Document 7: JP-A-2001-164109